Require C++14 and CMake 3.1

See merge request !79

- Avoid that defining points are aligned on a Plane
- Improve numerical stability of cross product

- cellIntervalToSlice more flexible

fix MPIEXEC_PREFLAG

See merge request walberla/walberla!80

Also remove some unneeded boost includes

Resolve "body filter to exclude bodies from PeVTKMeshWriter"

Closes #47

See merge request walberla/walberla!77

Resolve "Rename setMass function"

Closes #46

See merge request walberla/walberla!75

Dynamic refinement for pe module

See merge request walberla/walberla!52

Reworked Body Mapping

See merge request walberla/walberla!76

static calcMass and calcInertia functions are now available for all geometries

[API]: changed body mapping functions: removed most of them, added mapping-decider function, accomodated changes to test cases, added new mapping test case

CMAKE_FIND_ROOT to be able to find external libraries like openmesh
CMAKE_INSTALL_PREFIX for a possible later "make install"

Updated conda packages to cond build > 3.1.4 using host section
See https://conda.io/docs/user-guide/tasks/build-packages/define-metadata.html#host

Resolve "Add builds for GCC 7"

Closes #24

See merge request walberla/walberla!37

- fixed problem with new conda by using new compiler mechanism
- added test file that is checked before upload
- open mesh recipe fixed
- force downgrade for conda-build, new tested version(3.2.1) of conda-build leads to an empty
  package and then failing test.. perhaps it is fixed with newer conda-build versions?